The use of digital signals for the transmission of various types of information continues to grow in importance.
Digital modulation involves the mapping of bits into symbols, filtering the symbols into desired pulse shapes, and the translation of the baseband pulses onto a carrier signal for transmission. The mapping of bits into symbols involves, e.g., collecting N bits and mapping those bits into 1 of 2.sup.N signal amplitude and phase values. As an example, consider QPSK (Quadrature Phase Shift Keying) which maps 2 bits into 1 of 4 amplitude and phase values as illustrated in FIG. 1.
Pulse shaping is used to limit the bandwidth of the transmitted signal to the desired channel bandwidth. It can be accomplished by a digital filtering operation and is often implemented as an FIR (finite impulse response filter). Because the channel bandwidth must be wider than the symbol transmission rate, the rate at which the pulse shaping filter operates must be greater than the symbol rate. Generally, it must be at least twice the channel bandwidth but is often greater than that so as to be an integer multiple of the symbol rate, for example 2.times. or 4.times. the symbol rate. The normalized frequency characteristics for a representative 4.times. low pass pulse shaping filter is illustrated in FIG. 2.
In various known modulators, the translation to a carrier frequency is often carried out after the signal is converted from a digital to an analog signal. In such embodiments, a mixing operation is performed in the analog domain to convert the information signal up to the carrier frequency.
Such known systems have the disadvantage of requiring analog mixers along with other associated analog circuitry. In view of the high degree of reliability of digital integrated circuits as compared to analog system components, there are advantages in moving to a design implemented using all or almost all digital as opposed to analog circuitry.
One particular known modulator which is described in U.S. Pat. No. 5,412,352 is illustrated in FIG. 3. The modulator of FIG. 3 requires a single frequency translation from a digital baseband signal to a selected carrier frequency. This frequency translation is performed in the digital domain.
The modulator of FIG. 3 includes a symbol mapping circuit 72, a pulse shaping circuit 73, an interpolator 74, first and second mixers 75, 76 for mixing the digital I and Q signals output by the interpolator 74, an oscillator 78, a phase shifter 77, a summer 80, and a D/A converter 79. The oscillator 78 and mixers 75, 76 are located after the interpolator 74.
In the known modulator 70, in order to create a digital signal at the relatively high frequency of the carrier signal, e.g., 5-40 MHz, an interpolator 74 is placed between the output of the pulse shaping circuit 73 and the mixers 75, 76.
Unfortunately, each of the known systems suffers from the disadvantage of either performing a mixing operation to the carrier frequency in the analog domain or, as in the case of the modulator illustrated in FIG. 3, having to provide digital mixers 75, 76 capable of operating at the ultimate sampling frequency. Because of the relatively high carrier frequency, e.g., 40 MHz, the cost of implementing such mixers 75, 76 can be prohibitive in certain applications. This is due in large part to the cost of high speed multipliers needed to implement the mixers 75, 76.
Accordingly, there is a need for a digital modulator capable of mixing a signal to a carrier frequency in the digital domain that can be implemented at a reasonable cost. Furthermore, there is a need for methods and apparatus for implementing and controlling, at a reasonable cost, digital filters used to implement a digital modulator.